hclsyntax: Recovery of incomplete `ConditionalExpr`

## Background

Similar to some earlier issues, we found some more edge cases while implementing support for `ConditionalExpr` in the Language Server.

https://github.com/hashicorp/hcl-lang/pull/326

- incomplete `ConditionalExpr`
- `attr = ? var.foo : var.bar`
- `attr = var.foo ? : var.bar`
- `attr = var.foo ? var.bar :`
- `attr = var.foo ? `
- "impeding" incomplete `Traversal` (with trailing dot) inside otherwise complete `ConditionalExpr`
- `attr = var. ? var.foo : var.bar`
- `attr = var.foo ? var. : var.bar`
- `attr = var.foo ? var.bar : var.`

As with other reports related to recovery from incomplete configuration, the context is - again - enabling completion. Users would reasonably expect to be offered relevant completion in the above contexts, because it is exactly the nature of _completion_ to _complete_ the _incomplete_ configuration.

## Proposal

Regarding the incomplete `Traversal` specifically - I remember seeing other contexts where the trailing dot is simply ignored and the outer expression is recovered and so is the attribute as whole. I am therefore a little more hopeful we could do something about these edge cases.

The other ones like `attr = ? var.foo : var.bar` I would think could still be reported as `ConditionalExpr` but with either `nil` or some kind of empty expressions (I've seen `LiteralValueExpr{}` used before in similar contexts, such as `attr =`).

The only incomplete expression I can see being really difficult or maybe impossible to deal with meaningfully is the one which is missing a control character `:` (`attr = var.foo ? `) because there could be lack of confidence in what kind of expression is it even supposed to be.